Tafazzin (TAZ) is a mitochondrial transacylase that can catalyze the transfer of acyl chains from phosphatidyl choline to cardiolipin (CL), remodeling monolysocardiolipin (MLCL) to tetralinoleoyl cardiolipin (L4CL) (see Houtkooper R H, et al. (2009) Biochim. Biophys. Acta 1788, 2003-2014 and Xu Y, et al. (2003) J. Biol. Chem. 278, 51380-51385). Mutations in TAZ can result in impairment of lipid metabolism (see Aprikyan A A and Khuchua Z. (2013) Brit. J. Haematol. 161, 330-338) leading to mitochondrial dysfunction, which can be manifested clinically, for example, in highly energetic tissues such as the heart and skeletal muscle (see id. and Khuchua Z, et al. (2006) Circ. Res. 99, 201-208).
Studies have shown that CL, a structurally unique phospholipid component of the inner mitochondrial membrane, can provide functional support for electron transport chain complexes (see Kiebish M A, et al. (2013) J. Lipid Res. 54, 1312-1325 and Pfeiffer K, et al. (2003) J. Biol. Chem. 278, 52873-52880). In the absence of CL, respiratory supercomplex formation can be hindered (see McKenzie M, et al. (2006) J. Mol. Biol. 361, 462-469 and Zhang M, Mileykovskaya E, Dowhan W (2002) J. Biol. Chem. 277, 43553-43556) and individual complex activity can be decreased (see Zhang M, Mileykovskaya E, Dowhan W. (2005) J. Biol. Chem. 280, 29403-29408). Additionally, disturbances in the acyl chain composition of CL have been linked to impaired mitochondrial respiratory function (see Xu Y, et al. (2003) J. Biol. Chem. 278, 51380-51385), possibly through alteration in membrane dynamics (see Baile M G, et al. (2014) J. Biol. Chem. 289, 1768-1778).
It has been reported that cardiomyocytes (CMs) derived from induced pluripotent stem cells (iPS cells) containing targeted TAZ mutations demonstrate substantially normal ATP stores but increased reactive oxygen species (ROS) (Wang G, et al. (2014) Nat. Med. 20, 616-623). ROS have been implicated in the pathogenesis of cardiac hypertrophy and regulation of excitation-contraction coupling through effects on specific signaling pathways such as ERK, AKT, and PKA (see Sag C M, Santos C X, Shah A M (2014) J. Mol. Cell Cardiol. 73C, 103-111). Mitochondrial ROS, in particular, have been associated with angiotensin H-induced hypertrophy and heart failure associated with Gαq signaling (see Dai D F, et al. (2011) Circ. Res. 108, 837-846). Excessive ROS have also been linked to apoptosis (see Murphy M P, et al. (2011) Cell Metab. 13, 361-366).
Tafazzin orthologs are present in a wide variety of model organisms, indicating tafazzin's importance in mitochondrial biology, and providing the opportunity for the study of tafazzin function in organisms as diverse as, but not limited to, yeast, drosophila, and mice. In humans, the tafazzin gene is expressed via multiple splice variants, at least two of which have enzymatic activity. In experimental models of heart failure, such as the spontaneously hypertensive rat model, and in tissue samples collected from patients with heart failure, tafazzin expression and remodeled cardiolipin levels are reduced, suggesting that TAZ may play a common role in multiple forms of heart failure (see Sparagna G C, et al. (2007) J. Lipid Res. 48, 1559-1570). Since striated muscle cells generally require ATP for contraction, defects in mitochondrial function may lead to diminished ATP stores and consequent contractile dysfunction, as suggested previously (see He Q (2010) Am. J. Physiol.-Heart C. 299, H210-H216).
Barth syndrome (BTHS) is an X-linked disorder that is often characterized by mitochondrial functional impairment resulting in cardiac and skeletal myopathies, cyclic neutropenia, hypotonia, and 3-methylglutaconic aciduria (see Barth P G, et al. (2004) Am. J. Med. Genet. Part A 126A, 349-354). BTHS mitochondria can also appear morphologically abnormal and mitochondrial respiration can be impaired. TAZ has been linked with BTHS (see Xu Y, et al. (2006) J. Biol. Chem. 281, 39217-39224). Clinically, the cardiomyopathy observed in BTHS patients is primarily dilated cardiomyopathy and usually presents in the first year of life, although noncompaction cardiomyopathy has also been observed both clinically (see Bleyl S B, et al. (1997) Am. J. Med. Genet. 72, 257-265) and experimentally (see Phoon C K L, et al. (2012) J Am. Heart Assoc. 1, jah3-e000455-jah000453-e000455). Skeletal muscle weakness is also present early in life in BTHS patients, and neutropenia can vary, ranging from mild depression to complete absence, often in the same patient and often triggered by infection. Mortality was reportedly highest in infancy and childhood in early studies, but more recently, better diagnosis and treatment has improved survival, although few BTHS patients survive beyond middle age. At present, no specific therapy is available for BTHS patients.
The role of TAZ mutations in the pathogenesis of cardiomyopathy in BTHS has traditionally been ascribed to ATP deficiency due to the inefficient mitochondrial respiration observed in TAZ-deficient cells; however, it has been suggested that excessive production of reactive oxygen species (ROS) may be a more likely cause (see Wang G, et al. (2014) Nat. Med. 20, 616-623). Reduced levels of TAZ have also been observed in other forms of experimental heart failure and in clinical samples from patients with heart failure, raising the possibility that abnormalities in TAZ-dependent mitochondrial function may serve as a final common pathway for heart failure (see Sparagna G C, et al. (2007) 48, 1559-1570).
At the cellular level, hearts from BTHS patients can demonstrate morphologically abnormal mitochondria (see Neustein H B, et al. (1979) Pediatrics 64, 24-29) while fibroblasts can demonstrate impaired mitochondrial respiration due to deficiency of Complex III and IV activity (see Barth P G, et al. (1996) J. Inherit. Metab. Dis. 19, 157-160). Cultured fibroblasts can also demonstrate severe depletion of tetralinoleoyl cardiolipin (see Valianpour F, et al. (2003) J. Lipid Res, 44, 560-566 and Valianpour F, et al. (2002) J. Pediatr. 141, 729-733) that could be rescued by linoleic acid supplementation. Similarly, heart tissue can also demonstrate deficiency of tetralinoleoyl cardiolipin in the absence of functional tafazzin (see Schlame M, et al. (2002) Ann. Neurol. 51, 634-637).
A mouse model of inducible tafazzin deficiency has been developed, which can recapitulate many of the clinical manifestations of BTHS (see Acehan D, et al. (2011) J. Biol. Chem. 286, 899-908; Phoon C K L, et al. (2012) J Am. Heart Assoc. 1, jah3-e000455-jah000453-e000455; and Soustek M S, et al. (2011) Hum. Gene Ther. 22, 865-871). Characterization of this mouse model has revealed defects in cardiolipin modification, mitochondrial morphology, and both cardiac and skeletal muscle function. At baseline, however, these mice do not generally demonstrate any appreciable, or substantially appreciable, cardiac dysfunction until the age of 8 months, suggesting that additional stress may be required to trigger the onset of heart failure.
Nutritional supplementations with carnitine and linoleic acid have been promoted as potential treatments for BTHS, but results have been disappointing. Skeletal myopathy is generally difficult to treat. Heart failure in BTHS patients can be treated supportively, with diuretics, afterload reduction, beta blockade, and mechanical support as a bridge to transplant in severe cases, which are standard treatments for most cases of systolic heart failure. Infections resulting from neutropenia are treated with antibiotics. Although these interventions can alleviate symptoms and possibly improve outcome in BTHS patients, they do not treat the underlying metabolic disorder.